1. Field of the Invention
The present invention relates to a brushless direct current motors, and in particular, to the stator coil driver circuitry for the brushless direct current motor, and still more particularly to a slew rate control circuit for the driver circuit.
2. Technical Background
Brushless direct current motors (DC motors) are commonly used in disk drives, tape drives, video cassette players, and the like and are typically under stringent requirements regarding their performance in these applications. During the phase commutation of such a motor, which is achieved by switching off the current in one stator coil while at the same time switching on the current in another coil, current ripple has been a problem. It is known in the art that the commutation should be performed when the back electromotive force (BEMF) on the two coils is equal and the torque provided by each coil is also equal. Torque ripple has been a problem during commutation which results in undesirable acoustical noise produced by the motor and unnecessary wear on the motor.
FIG. 1 illustrates the typical architecture of a brushless DC motor as is known in the art. This architecture includes a motor 12, a commutator 20, a driver 22, and a voltage supply 24. The motor includes a rotor 14, a stator 16, and hall effect sensors 103. (Although this block diagram shows hall effect sensors, it is also known in the art to use the BEMF of a floating coil to determine the position of the rotor instead of using hall effect sensors.) The stator 16 includes stator coils 26a, 26b, and 26c configured in a wye configuration. In operation, the commutator block 20 sequentially selects the appropriate stator coil driver circuit in driver block 22 to drive current into or out of stator coils 16a, 16b, or 16c, as is known in the art. Hall effect sensors 103, in combination with rotor 14, provide the position information necessary so that the commutator circuit 20 can commutate the driver circuit 22 at the appropriate time. The operation of a typical disk drive is more fully described in U.S. Pat. Nos. 5,017,845, 5,172,036, 5,191,269, 5,221,881, and 5,306,988, and are fully incorporated into this specification by reference.
FIG. 2 shows a prior art circuit, which is described in detail in U.S. Pat. No. 5,191,269, used to reduce commutation ripple. This circuit reduces commutation ripple by using a combination of voltage slew-rate control on the phase which is turning off and a fast closing of the current loop through the phase which is turning on. More specifically, FIG. 2 shows stator coils 26a, 26b, and 26c in a wye configuration. A low side driver circuit for stator coil 26a is shown as including switch 56, current source 72, amplifier 70, capacitor 76, transistor 38. A low side driver circuit for stator coil 26b is shown as including switch 62, current source 84, amplifier 82, capacitor 88, transistor 44.
In FIG. 2, stator coil 26a is being turned off while stator coil 26b is being turned on. Upon the commutator causing the first switch 56 to open while causing the second switch 62 to close, the slew-rate control controls the rate of turn-off of the current flowing in the one phase while the current sensing resistor senses the sum of the current flowing in transistors 38 and 44. The current sensing resistor imposes a feedback voltage indicative of the summed current of transistors 38 and 44 at the inverting input of the operational amplifier 50. The operational amplifier 50 produces a voltage at the output representative of the voltage difference between the predetermined voltage of the voltage source and the feedback voltage, whereby the voltage difference at the gate of the second transistor controls the rate of turn-on of the other phase so that the total current in the phase is maintained constant through the commutation and is equal to Vin divided by the resistance of the sensing resistor 30.
The drawbacks to this approach are:
1. The technique does not work in pulse width modulation (PWM) mode since the slew-rate control is killed by the PWM operation, thus preventing a smooth current transition. PA1 2. The fast turn-on of the phase controlling the current generates EMI, unless some circuitry is added to control turn-on slew-rate, potentially at the expense of current stability. PA1 3. The circuit requires some extra circuitry to minimize delays when the commutations are effected from a "saturated" condition (i.e. gate overdrive) which is typically used when a series device is controlling the current such is in high power applications. PA1 4. The overall predriver circuitry is rather complicated, partially because of all the above patches.